1. Technical Field of the Invention
The present invention relates to an implantable dual chamber heart stimulator comprising a stimulation threshold detector, an AV-interval generator, and a controller.
2. Description of the Related Art
To reduce the energy consumption of heart stimulators, an automatic threshold search function is used to maintain the energy of the stimulation pulses at a level just above that which is needed to effectuate capture.
FIG. 1 discloses an intracardiac electrogram (IEGM) illustrating the principles of threshold search algorithms according to established standard prior art (see e.g., U.S. Pat. No. 5,476,487) and which principles the present invention addresses. FIG. 1 shows atrial A and ventricular V stimulation pulses, as well as a high output backup pulse BU delivered if loss of capture (LOC) occurs. As can be seen in complex 3, the pre-programmed AV-interval is prolonged with an atrial pulse A when an LOC occurs (complex 2). The reason for that is to wait for any intrinsic event if the first LOC was the result of a fusion beat. In this case there is no intrinsic activity and the LOC was not a result of a fusion beat but was due to a changed stimulation threshold of the heart tissue, and a stimulation threshold search is initiated. During the threshold search, the pre programmed AV-interval is shortened to “‘AV-short” to override any intrinsic heart activity. The ventricular stimulation amplitude is successively stepped up by a predetermined amplitude step of, e.g. 0.1-0.3 V, and each unsuccessful ventricular stimulation pulse is followed by a back-up pulse. As an alternative, the ventricular stimulation amplitude may start at an amplitude above the stimulation threshold and then successively be stepped down until non capture occurs. This is performed until the stimulation threshold is detected, i.e. capture is detected from the ventricular stimulation pulse, and the stimulation pulse amplitude is then set to a value that equals the stimulation threshold plus a working margin, e.g. 0.3 V.
For the purposes of the discussions and definitions below relating to the invention, the present invention is directed to a dual chamber heart stimulator having, among other things, an inhibiting function such that, using the commonly accepted terminology, the AV interval could also be started by an intrinsic atrial heart activity (a P-wave), and the started interval is then designated a PV interval. Thus, instead of using the nomenclature “PV/AV-interval” to describe this, the nomenclature “AV-interval” or “AVI” is used in this application whenever there is no need to distinguish between the two. The AVI so far discussed is thus artificial in the sense that it is controlled by an AV-counter in the pacemaker. However, there is also a natural AV interval or AV conduction time, which is related to the physiological conduction pathway in the heart. This interval is also referred to as the PR/AR interval as it starts with either a P-wave or an A-pulse and ends with the natural ventricular depolarization indicated by an R-wave. Whenever necessary for clarity, the more specific terms just discussed will be used.
U.S. Pat. No. 5,766,229 discloses another capture verification method and apparatus for a single chamber or a double chamber implantable pacemaker utilizing heart rhythm stability measurements to minimize the likelihood of fusion.